Imaging electron energy filters, also known as electron filters, energy filters or electron energy spectrometers are used in transmission electron microscopes to improve contrast of the image of the specimen by selecting electrons of a particular energy range as well as to register element distribution images and filtered electron diffraction diagrams. An alpha filter is disclosed in U.S. Pat. No. 4,760,261 and an omega filter is disclosed in U.S. Pat. No. 4,740,704.
Such filters include magnets having pole pieces above and below a symmetry plane wherein the center axis of the electron beam is guided. Homogeneous magnetic fields are generated between the pole piece pairs which deflect the electron beam several times so that its center axis in the alpha filter corresponds approximately to the lowercase Greek letter alpha and, in the case of the omega filter, to an uppercase Greek letter omega.
The deflecting radii of the electrons are dependent upon their velocity in the magnetic fields. For this reason, an energy spectrum of the transmitted electrons is obtained downstream of the filter of which a region can be masked.
Such electron energy filters mostly include several separate magnetic fields or deflecting regions which are separated from each other by more or less large spaces having field free regions. In these regions, the electron beam runs in a straight line between shielding plates. In the two patents referred to above, an embodiment is described wherein the pole pieces define a common part with the shielding plates such that the filters are assembled from two mirror-image symmetrical parts in which the pole pieces and the required channels for the electron beam are machined. Although a very good stability of the filter is obtained in this way in a simple manner, it has however been shown that such a mechanical assembly is quite expensive if the required precision in the mechanical dimensions is to be maintained.